Operational amplifiers are widely used in the electronics industry because of their many excellent circuit characteristics including high open loop gain, high input impedance, and low output impedance. General applications of the operational amplifier include circuit configurations such as voltage and current amplifiers, differentiators and integrators, active filters, oscillators, and analog to digital and digital to analog converters. To realize these different circuit configurations, operational amplifiers are used in conjunction with positive and or negative feedback in combination with passive and or active elements.
An operational amplifier is also widely used to function as a voltage comparator, wherein typically, a reference signal is applied to the inverting input and the voltage to be compared is applied to the noninverting input. If the magnitude of the voltage to be compared is greater than the magnitude of the reference signal, the output of the comparator substantially equals the positive supply voltage. If the magnitude of the voltage to be compared is less than the magnitude of the reference voltage, the output of the comparator substantially equals the negative or ground supply voltage. An inverted voltage comparator may be provided by simply transposing the signals at the inverting and noninverting inputs. Using the operational amplifier as a voltage comparator requires no external components or feedback, and its output only has two states of high and low.
The operational amplifier as utilized in the realization of a variety of circuit functions may be manufactured in bipolar or Complementary Metal Oxide Semiconductor (CMOS) technology or some combination thereof. The CMOS implementation is desirable for its high packing density and low power consumption characteristics. Also, operational amplifiers are increasingly being integrated onto chips which merge digital and analog functions together with an increasing number of devices.
A typical problem associated with operational amplifiers is that of an offset error voltage. This type of error appears as deviations in the expected output due to imbalances in the input stage. This is further due to statistical deviations between the devices of the inverting and noninverting inputs. While this offset error is correctable by null offset adjustment, it requires additional circuitry and adds to the manufacturing time. A secondary problem often associated with null offset adjustment is a resulting decrease in the operational amplifier's input common mode range.
Early operational amplifiers provided additional pins to which a variable resistor could be connected for making a null offset adjustment. This had the disadvantages of requiring additional pins on the operational amplifier integrated circuit therein increasing its cost, requiring the additional resistor or trim potentiometer, and increasing the size of the printed circuit board. Also, the operational amplifier and trim potentiometer had different temperature coefficients and would not track over temperature changes causing the null offset to drift. An improved method of null offset adjustment is to trim a resistor on the integrated circuit itself by a laser beam while applying input signals and monitoring the output. This method improves the temperature coefficient problem but requires the additional circuitry, expensive laser beam equipment, reduction in input common mode range, adjustment time, and text fixtures. Due to the aging of the semiconductor, null offset adjustments may drift with no means available to readjust.
If fully differential operational amplifiers are available, versus single ended, an improved method of reducing the null offset error is available by using chopper stabilization techniques. This technique uses a chopper at the input of a first fully differential operational amplifier in order to alternate the application of the inverting and noninverting input signals (transpose) to the first fully differential operational amplifier at some frequency which is at least twice the frequency of the input signal frequency. The outputs of the first fully differential operational amplifier are input into a second chopper for alternating the application of the output signals to a second fully differential operational amplifier. The offset error is reduced because the error is reversed on each transposition of the input and output signals, therein allowing the error due to device mismatches to cancel out. While this technique reduces the offset error, it has the disadvantages of limiting the bandwidth to typically less than 50 kilohertz, introducing switching noise, and requiring fully differential operational amplifiers.
Thus, what is needed is a single ended operational amplifier having an improved offset voltage using chopper stabilization techniques which reduces switching noise and improves bandwidth.